OK, try this scenario...
Enclosure has a UL label/listing as a pull or junction box, based on the UL White Book? If that is/was the case then by the 'book' he's technically correct.
Did the enclosure you used have a backplate? Mounting studs?
Did you have a Sq D (or other mfg) that has a 'factory' enclosure in their catalog?
From your last post, sounds like that AHJ may have had a UL White Book with him
